Conventionally, a rotation speed detection device for detecting the rotation speed of a compressor by utilizing the variation of a magnetic field is publicly known. In this rotation speed detection device, a detection unit (a sensor) for detecting the variation of a magnetic field is provided with a casing of the compressor. The rotation speed detection device measures the rotation speed of the compressor by detecting the variation of a magnetic field due to approach of blades that are detected bodies. An example for this is shown in Patent Literature 1.
With respect to the rotation speed detection device of Patent Literature 1, the detection unit is arranged such that the detection unit is exposed to an intake compression passage inside of the compressor so as to detect the approach of the blades accurately. The inside of the intake compression passage is in a high-temperature and high-pressure state due to compression heat that is generated in a process in which intake air is pressurized and compressed by the blades. As such, in the conventional rotation speed detection device, dissolved loss due to intake pressure or compression heat and decrease of detection accuracy can be induced depending on the operation state of the compressor and the position of the detection unit arranged inside of the intake compression passage.